The present invention relates to a thin infrared lens, especially to an assembled infrared optical lens.
In the field of the infrared sensing system, especially in a thermal imager, the sensor and the infrared optical system are two most important elements. Thanks to the development of the two-dimensional matrix sensing element, the cost of the infrared sensing system has been greatly decreased. However, in order to make it possible for the infrared sensing system become popular, it is necessary to reduce the manufacture costs for the optical system of the infrared sensing system.
The infrared optical system may be divided into three groups. They are the refraction type, the reflection type and the diffraction type.
In a reflection type infrared system, when a wider viewing angle is desirable, it is necessary to provide a larger space for the optical system, if the non-coaxial light system is employed; if the coaxial light system is used, a portion of the incident light would be blocked, such that the sensitivity of the optical system will be damaged.
In contrast, the refraction type infrared optical system uses a coaxial light system and is easy to manufacture and use. The structure of a refraction type infrared system may be very compact. In such a system, the aberration of the system may be calibrated by using a spherical lens or by adding number of lens thereto. As a result, the refraction type infrared system is advantageous over the reflection type infrared system.
The refraction type infrared optical system, however, is greatly limited by the materials of the optical elements.
It is known that materials that allows mid to far infrared band to penetrate are limited. Materials that may be used in a refraction type infrared optical system include Ge, Si, ZnSe and poly ethylene (PE) plastics. Among these materials, Ge is most commonly used. It is also known that, if preparation costs of the infrared optical system shall be saved, a thin Fresnel fraction lens will be first choice.
As known to the public, when a convex lens is thinned by dividing its curvature with concentric circles, the lens may provide the same focusing effects. Although the notches in the lens may cause scattering and damage the quality of the image, such thinned lens is suited in an infrared optical system, since the spatial resolution of the sensor matrix of the infrared sensing system is always low. As to the design of the concentric circles, they may be concentric circles with diameters at a constant difference or a diminished difference. In a Fresnel lens of the constant difference type, the outer circles have greater height and deeper grooves, relatively to the inner circles. In a Fresnel lens of the diminished difference type, all grooves may have the same depth and number of concentric circles in the central area is less than that in the outer area.
In the preparation of a lens, the more circles it has, the less its thickness can be but the greater the scattering of light beams may be. In a diminished difference type Fresnel lens, number of circles in the central area is less than that in a constant difference type lens in the same area. The imaging quality of a diminished difference type lens is better than that of a constant difference type lens. However, due to the limitations of preciseness in preparing the outer circles, it is still difficult to make the central area as thin as desired.
The thickness of a Fresnel lens is always far less than that of a traditional refraction type lens. As a result, it is not necessary to prepare a Fresnel lens with high-transmission rate but expensive materials. It is possible to produce in quantity Fresnel lenses with PE plastics by injecting or casting the plastic materials to molds of the lens. In order to improve the hardness, thermal resistance and transmission rate of PE materials, suited additives may be added and the hydrogen atoms contained in the plastic materials may be removed. Such lenses are commonly used in the human body temperature sensing lamp switch and security systems. A tens array may be used in combination with pilot sensors, such that moving objects may be sensed.
Another approach to prepare a thin lens is the diffraction infrared lens. A silicon or germanium lens with binary steps may be prepared by etching a wafer or forming a thin film on a wafer. Such a binary optical lens is called a fresnel phase plate. In order to improve its diffraction efficiency and its focusing effects, adding the number of steps is required. In order to improve its chromatic aberration in the infrared band, it is possible to use a hybrid lens comprising a diffraction lens and a refraction lens.
Although the above-said approaches are able to prepare a thin infrared lens, they inherit different problems. The PE plastic lens is low in cost but is soft, tends to deformation and is less resistant to high temperature. A PE plastic lens may not be used as a preposition optical element. On the other hand, the diffraction lens is hard in material but is difficult to prepare, because the optical preciseness in the outer area of the lens is highly required. In addition, because of the relatively great number of circles, the optical scattering of the lens may go beyond applicable requirements.
It is thus a need in the industry to provide a novel thin infrared lens that can provide improved transmission rate for infrared lights and improved focusing effects and that can satisfy the requirements in thickness and hardness.
The objective of this invention is to provide a novel thin infrared lens.
Another objective of this invention is to provide a thin infrared lens that can satisfy the requirements in thickness and in hardness at the same time.
Another objective of this invention is to provide a thin infrared lens that is easy to produce under a relatively low cost.
Another objective of this invention is to provide a thin infrared lens comprising an infrared lens and a hard thin substrate.
According to this invention, a novel thin infrared lens is disclosed. The thin infrared lens of this invention comprises an infrared lens and a hard and thin substrate that allows infrared lights to penetrate. The infrared lens may be a Fresnel refraction lens, prepared as a thin film by injection or casting. The infrared lens is attached to the thin substrate to form a lens assembly. The infrared lens may be pressed or adhered to the thin substrate or enveloped by the thin substrate to form the assembly.
The above and other objectives and advantages of this invention may be clearly understood from the detailed description by referring to the following drawings.